Add CHANGELOG.

Author: florijanstamenkovic

CHANGELOG.md

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+# Changelog
+
+## Unreleased
+
+### Changed
+* `Client.execute` now automatically extracts the 'data' value from the
+returned `dict`. This *breaks* existing code that directly uses the
+`Client.execute` method.
+* Major code reorganization, naming and test improvements.
+
+### Added
+* `Project.upsert_review_queue` method.
+* `Project.extend_reservations` method.
+* `Label.created_by` relationship (To-One User).
+* Changelog.
+
+### Fixed
+
+## Version 2.2 (2019-10-18)
+Changelog not maintained before version 2.2.